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Why did Heidegger think demanding proof of an external world was a philosophical mistake?

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Marble bust of Aristotle with a beard and draped robe, showing detailed facial features and wavy hair, set against a dark background. — metaphysics Detail of a Roman copy (2nd century bce) of a Greek alabaster portrait bust of Aristotle (c. 325 bce); in the collection of the Museo Nazionale Romano, Rome.

Martin Heidegger argued that demanding a proof for the existence of an external world is a fundamental philosophical mistake because it relies on a flawed, Cartesian conception of human existence.12

In his work Being and Time, Heidegger criticized the modern tradition—originating with René Descartes—for positing a radical separation between an "inner" world of subjective mental experience and an "outer" world of objective material things. This framework incorrectly assumes that humans are isolated subjects who must use mental representations to bridge the gap to an external reality.314

Heidegger replaced this model with the concept of Dasein, or "being-in-the-world," which describes human existence as inherently involved with and caught up in the world from the start. Because humans are always already "there" in the world, the skeptical demand for a proof is based on a distorted abstraction. Heidegger famously remarked that the "scandal to philosophy" is not that such a proof has not been given, but that philosophers continue to expect and attempt it.312